The use of certain glass-ceramics consisting predominantly of lithium oxide and silicon dioxide as materials for dental restorations has been suggested. For example, it has been reported (Hench, L. L. et al., "Glass-Ceramic Dental Restorations", presented at meeting of International Association of Dental Research; Chicago, Illinois; Mar. 19, 1971) that a glass-ceramic consisting of 33 mole percent lithium oxide and 67 mole percent silicon dioxide exhibits a mechanical strength which compares favorably with that of amalgam, silicate, acrylate and fused porcelain dental restorative materials. Glass articles of the 33 mole percent lithium oxide-67 mole percent silicon dioxide composition may be readily prepared from the melt by casting and cooling in commercially available dental laboratory investment molds commonly used for metal castings, and then devitrified by further heat treatment. The ability to use these commonly available molds is highly advantageous from an economic standpoint since the need to purchase and operate specialized equipment is avoided. However, the effectiveness of this Li.sub.2 O-SiO.sub.2 glass-ceramic composition as a dental restorative material is limited by its inadequate chemical durability, i.e., its inadequate resistance to chemical attack in the physiological setting of its use.
Glass-ceramics from the lithium oxide-aluminum oxide-silicon dioxide and lithium oxide-zinc oxide-silicon dioxide systems have also been suggested as materials for dental restorations (MacCulloch, W. T., "Advances in Dental Ceramics", Brit. Dental J., 124, 361-365 (1968); Chu, G.P.K., "Dental Porcelain: The State of the Art", University of Southern California Dental School, Yamada, H. N., ed., 35-40 (1977); Kasloff, Z., ibid., 241-244). The lithium oxide-aluminum oxide-silicon dioxide system is generally superior to the lithium oxide-silicon dioxide system in terms of chemical durability, but both systems exhibit such high melt viscosities that casting temperatures of about 1350.degree. to about 1400.degree. C. are required. The employment of such high casting temperatures is wasteful of energy and can cause significant Li.sub.2 O instability problems.
Additionally, glass-ceramics consisting predominantly of Li.sub.2 O and SiO.sub.2, and having relatively low ratios of silica to alkali, may possess cracks arising from the local volume change during thermal crystallization. These cracks contribute to low fracture toughness and prevent the glass-ceramics from realizing their maximum potential mechanical strength.